net: lib: tls_credentials: Rename TLS_CREDENTIAL_SERVER_CERTIFICATE

TLS_CREDENTIAL_SERVER_CERTIFICATE credential type is misleading, as in
fact it just represents a public certificate, it does not matter if the
certificate belongs to a server or a client. And actually, it was
already used in-tree for clients as well, for example in LwM2M.

Therefore rename the credential type to a more generic
TLS_CREDENTIAL_PUBLIC_CERTIFICATE and deprecate the old one.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>